Puzzled by browser redirects

We have several computers on a home network. We have noticed in the last couple of weeks that when trying to access the U.S. sites of Costco and Ticketmaster that sometimes, but not all the time, we have been redirected to Mexican sites of the same companies. We have tried to check for malware and rootkits but have not uncovered anything. Is there something else that could be causing this behavior? We have one PC running Windows 7 and Linux (from a USB drive) and another PC running Windows 8. This redirection has occurred using Google Chrome, Firefox, and Internet Explorer.

I would have thought that using the direct URL for them from the UK would have given me more local sites to my location as a Google search for Costco and Ticketmaster does.

Both sites come up in English (USA) for me, however, on your Costco site, hit the Mexican flag and change it back to American. It should stay that way
the next time you access it. If it doesn't, clean your browsers cache with CCleaners most aggressive settings and repeat, then check through your browser's add-ons section.
Do a complete reboot after.

DRIVE IMAGINGInvest a little time and energy in a well thought out BACKUP regimen and you will have minimal down time, and headache.

Thank you to everyone for all of these suggestions. I will try them and report back.

Regarding CLiNT's first question, the Costco URL was www.costco.com. I cannot provide the particular Ticketmaster URL that triggered issues, as it was in the ticket payment section and we did not save it.

The ways I get unusual locations for websites which use location information are two:

Either In use Ghostery and Firefox with Linux, which masks my location information for my computer, but not for the nearest Internet Gateway used by my ISP at any moment
OR
I use an anonymizing service, which deliberately sends false location information through my browser. then I am located as being in (and get driected to) the website for the locality the anonymizer is set for. (In this way I can access media content which has regional restrictions. I believe this practice to be perfectly legal.)

Both conditions result in either no information coming from my computer to be used for location tracking, or false information, which then results in false location by the website.

If your computer is not sending your location information, the website will do its best to guess where you are, using the nearest Internet Gateway being used by your ISP. In Los Angleles southward, it is possible (though unlikely) that an ISP might be using a Gateway located in Mexico during times of heavy traffic, locally or at the website. Telephone calls could be routed in a similar way, though this too is unlikely. DSL is more likely to redirect to a distant Gatweay than Cable or FiOS (fiber optic).

I don't know whether during times of very heavy traffic, any websites redirect traffic from LA to Mexico. This seems unlikely, but it may be happening these days.

None of these scenarios indicates that anything malicious or insecure is happening. If you are concerned, both Costco and TicketMaster let you manually reset your location at the site.